The present invention is concerned with a flexible or resilient composite comprising a polymeric matrix having a fibrous component dispersed therein for reinforcement. The composite is characterized by its high puncture resistance and other useful properties.
A great deal of research effort has been, and is currently being, expended towards developing composites of resins and fibrous materials which provide needed properties. Typically such composites may include any one or more conventional resins or other matrix material such as epoxy or polyester resins, reinforced with various types of fibers including, for example, glass or metal fibers or the like.
A useful discussion regarding composites appears in an article by Chou et al entitled xe2x80x9cCompositesxe2x80x9d appearing in Scientific American, October, 1986, Volume 255, No. 4, pages 193-203. The article describes a variety of different types of composites comprising fibrous materials dispersed in various matrix materials. The article notes that, in the case of a brittle, ceramic matrix material, a crack in the matrix may cause the reinforcing fiber to fail as well unless the bond between the matrix and fiber is quite weak. Normally, however, steps are taken to provide for maximum bonding between the matrix and fibrous component. This may be accomplished by appropriate selection of the matrix and fibers and/or by pretreatment of the fibers to provide physical or chemical bonding to the matrix.
As noted, a variety of fibrous components in various forms, e.g. metal, glass, polyester, etc. in the form of woven, non-woven or knitted fabrics, or as staple fibers or filament bundles, have been proposed for composite use. More recently such materials as aramid and extended chain polyethylene fibers (e.g. xe2x80x9cSpectraxe2x80x9d fibers) have been proposed for use in composites. However, as far as can be ascertained, all such previously disclosed uses have required adhesion between the fibrous component and the matrix to provide useful flexible or resilient composites.
The present invention is based on the finding that a highly useful resilient or flexible composite can be obtained by combining a resilient resin component and a fibrous component such that the resin encases or envelops the fibrous component with essentially no adhesion or bond between the two components. This is substantively different from prior composites where, as noted, bonding between the resin and fibrous components has been considered desirable, if not essential. In the present case, the resin and fibrous components are so chosen that any significant amount of bonding does not occur. As a consequence, the resin, which is itself resilient, can retain its resiliency while performing the matrix function. At the same time, the fibrous component adds strength and other desirable properties, particularly puncture-resistance, to the composite.
Particularly effective results are obtained by forming the resin matrix in situ about the fibrous component which may be in the form of staple fibers, continuous filament, non-woven, woven or knitted fabric.
In a preferred embodiment, the invention contemplates the use of ultra-high molecular weight, high tensile strength, high modulus extended chain polyethylene fibers as the fibrous component and flexible polyurethane formed in situ by positioning the urethane-forming components about the fibers and allowing the desired urethane-forming reaction to occur. Such fibers and resin matrix do not bond together, the non-bonding effect being aided by the highly lubricious nature of the polyethylene fibers. Polyester fibers may also be usefully employed with the flexible polyurethane matrix or the like as long as any significant chemical bonding between the matrix and fibrous component is avoided. According to the invention, the composite is essentially as resilient as the polyurethane itself until the composite is bent to the point where the fibers in the matrix are snubbed, i.e. the matrix contracts around individual fibers to affect a braking action on-the slippage between the matrix and fibers. Up to this point, the composite may be bent without causing tension on the encapsulated fibers which, in a sense, float within the resin matrix. However, when the bending of the composite is such that fiber snubbing or braking occurs, the fibers increase their reinforcing effect by coacting with fibers in proximity thereto so as to spread the load placed on the composite. Then, when the bending force is released, the energy stored up in the snubbed fibers facilitates the return of the composite to its prior dimensions. The composite thus, in essence, retains desired flexibility or resiliency of the resin component while being reinforced by, and otherwise benefiting from, the fibers.
It is to be noted that the manner in which the present composite functions on bending and release would not be possible if the fibers and matrix were physically or chemically bonded together. Thus, significant or intentional adhesion between the fibers and matrix restricts flexibility and the thus encased fibers might well break before sharing the bending load with other adjacent fibers. In the present case, the fibers do not change position before, during or after deformation with respect to the matrix. The fibers instead float unadhered within the matrix until the composite is bent to the point where the fibers are stubbed or squeezed in their position by the bent matrix, the energy stored in the thus stubbed fibers helping to spring the composite back to its original form when the bending force is released.
A wide variety of resilient polymeric materials may be used to provide the matrix for the present composite. Preferably, however, as noted above, the matrix comprises a flexible or resilient polyurethane which is formed in situ by application of the polyurethane-forming reactants about the fibrous component followed by reaction and curing. Typically, the polyurethane-forming reactants comprise (A) an aliphatic isocyanate, e.g. an isocyanate prepolymer such as isophrone diisocyanate, or diphenylmethane diisocyanate and (B) an aliphatic hydroxy component such as a polyester polyol or a mixture thereof with polypropylene glycol. Any conventional polyurethane-forming components may be used for this purpose provided the polyurethane reaction occurs at a temperature below the melting point of the fibrous component. Preferably, the polyurethane is formed by separately preheating the reactants (A) and (B) to a temperature of, for example, 30-60xc2x0 C. and applying these reactants about the fibrous component, the latter being positioned in a mold or otherwise supported at ambient temperature (18-32xc2x0 C.). The resulting in situ reaction is an exothermic one which should be controlled, if necessary, to keep the temperature well below the melting point of the fibers involved. Usually, for polyethylene fibers, the temperature will be kept below about 70xc2x0 C. while higher temperatures, e.g. up to about 120xc2x0 C. may be observed with low shrinkage polyester fibers.
Polyurethane matrix materials, however, are preferred because they tend to have good abrasion resistance and, in the case of aliphate urethanes, good UV resistance; and in the case of polyethers, good hydrolytic stability.
While polyurethane comprises the preferred matrix, it will be recognized that other resins which are resilient may be used. This includes, for example, vinyl resins, ethylene propylene polymers, epoxies and the like. A variety of fibrous components or mixtures thereof may be used for present purposes. However, as noted, it is preferred that this component comprise either polyester fibers or ultra high molecular weight, high tensile strength polyolefin fibers. Of particular preference are extended chain polyethylene fibers, e.g. fibers available as Spectra 900 and Spectra 1000, which have been found to be especially effective. Such polyethylene fibers have exceptionally high tensile strength and, because of this, a fabric can be made from this fiber that is more open than fabrics made from lesser-strength fibers at a given strength level. For example, a fabric woven of 1,200-denier polyester yarn in a 32 by 32 construction (that is, 32 warp and 32 filling yarns per inch) has virtually no xe2x80x9cwindowsxe2x80x9d, i.e. there are essentially no openings therein such as there are in woven window screen fabric. Such a closed polyester fabric is less strong than an open fabric woven of 1,200 denier yarn of Spectra 900 or 1000 fiber in a 10 by 10 construction (that is, 10 warp and 10 filling yarns per inch). The latter fabric is so open that it has a substantial xe2x80x9cwindowxe2x80x9d between each warp and filling yarn. This ability to make a strong fabric with xe2x80x9cwindowsxe2x80x9d, coupled with the lubricious or slippery nature of the polyethylene fiber, makes this fiber especially useful for present purposes.
It is also noted that xe2x80x9cSpectraxe2x80x9d fiber transmits load faster than the next most high performance fiber, aramid; nearly twice as fast. Accordingly, when one xe2x80x9cSpectraxe2x80x9d fiber in fabric form is subjected to a force, that fiber quickly marshalls its companion fibers, sharing their assigned reinforcement task at the rate of 12,300 meters per second. Consequently, a runner, for example, stepping on a nail with a shoe midsole made according to the invention brings a myriad of super-strong fibers, automatically and virtually immediately, into protective behavior.
The xe2x80x9cSpectraxe2x80x9d fiber is lighter, i.e. it has a lower specific gravity (0.97) than polyester (1.38) or aramid (1.44) or glass (2.6) or steel (7.0) and still outperforms the other fibers. This is consequential because it enables the present composites to exhibit maximum strength:weight properties.
It is noted that xe2x80x9cSpectraxe2x80x9d fiber tends to shrink at the boiling point of water and, at 250xc2x0 F., the fiber starts to weaken slowly but noticeably. As a consequence, the resin selected for use with these fibers should be formed at temperatures well below the boiling point of water, preferably, for example, at essentially hand-washing temperatures (below about 70xc2x0 C.).
The xe2x80x9cSpectraxe2x80x9d fibers or like extended chain polyolefin fibers do not bond to most resins. Steps have been taken in the past to improve the adhesion properties of these fibers, e.g. by corona or plasma treatment or by special adhesives, in order to form composites because it has been thought that such adhesion was essential. However, for present purposes, it is important that the fibers not adhere to the resin matrix in order to obtain a resilient composite. In this regard, some incidental physical adhesion may occur between the fibers and the matrix due, for example, to irregularities in the fiber surface. However, such incidental adhesion is not the sort which the invention intends to be avoided. The key thing is to avoid chemical adhesion between the matrix and fibrous components.
The present composite may be prepared in a variety of ways. For example, the resin or resin-forming components may be cast or nipped around the fibers or they may be sprayed among the fibrous component and allowed to set or, in the case where the resin is formed in situ, the resin-forming components are permitted to react and cure. Advantageously, two liquid reactant parts of the matrix resin may be preheated and mixed precisely under high pressure at the head of an airless spray gun. The thus atomized resin mixture is then deposited on and among the prepositioned fibers, these being placed in a suitable jig or mold or the like. Alternatively, the reactants may be co-sprayed with the fibers and any other optional components onto a suitable substrate or mold surface where the resin components react to form the matrix with the fibers dispersed therein. It will be appreciated that the substrate may be a fabric, paper, film, sheet, foil, metal or the like. In whatever method is used, the resin should envelop the reinforcing fibers or fibrous bundle without bonding thereto, it being noted that where a bundle of fibers is used, the resin may encapsulate the bundle while individual fibers thereof are not all encapsulated.
The fibrous component may be in any convenient form, e.g. as individual fibers, filaments, fiber bundles or non-woven, woven or knitted fabric. The term xe2x80x9cfibersxe2x80x9d is used for convenience herein although it is intended that the term embrace both staple fiber as well as continuous filament, cut to desired length, bundles thereof or fabrics based thereon.
The fibrous component may be used in random or oriented fashion depending, for example, on the properties desired in the composite. In the case of random configuration, staple fibers may be used. Such fibers tend to curl or bend to varying degrees when dispersed within the resin matrix. As an alternative, continuous filaments may be used in a random or scrambled fashion.
Preferably the fibers are used in fabric or bundle form and highly useful results are obtained when the fibers are straight and parallel in a given plane without crimp or meander. Multiple axes, each in a given plane, may also be used when the circumstances warrant. When fiber bundles are used, the bundles preferably include 10-1000 filaments with the bundles arranged in parallel in a common plane. Parallel layers of filaments may also be used with the filaments in each layer oriented at different angles to the filaments in adjacent layers. Preferably straight or uncrimped fibers are used because any force applied to such fibers instantly loads the fibers in tension or compression whereas crimped fibers need to be first straightened out by the applied force.
Orientation of fiber layers is used primarily to build strength in the desired direction(s). Advantageously, the fibers in each layer are in parallel, or essentially so, as this permits the packing of more fibers into a given volume. It is also preferred that the fibers be positioned so that their largest dimension (length) is parallel to the force it is intended to resist. The matrix serves to keep the oriented fibers in alignment, both individually and in bundle form, even through cycles of loading the composite in tension and compression and even though all of the fibers are not completely enveloped by the resin.